The weight of rock? (5)
I believe the answer is:
stone
I believe this is a double definition.
'the weight' is the first definition.
(stone is a unit of weight)
'rock?' is the second definition.
(I know that rock can be written as stone)
'of' acts as a link.
(Other definitions for stone that I've seen before include "Hard lump of mineral matter" , "Boulder" , "Rock; cherry seed" , "See 5" , "Olive pip" .)